1. Field of the Invention
This invention relates to a bone fixation device, and a process for retrofitting a fixation device in a hollow, intramedullary nail.
2. Discussion of the Prior Art
Intramedullary nails for treating fractures of long bones such as the femur, tibia, and humerus are well known in the prior art. One of the most popular intramedullary nails in use today was developed by Gerhard Kuntscher in the 1930's. This nail was extensively used by the Germans in World War II, and allowed them to quickly send many wounded Luftwaffe pilots with broken bones back into action without external braces. It is still used extensively today. Basically, the Kuntscher nail is a long, hollow stainless steel shaft having a cloverleaf cross-section, and a narrow slot running down its length. The back end of the nail terminates in an open mouth, while the front end of the nail terminates in a bullet-shaped nose to facilitate insertion into a fractured bone. In use, the intramedullary cavity of the bone is reamed out in accordance with standard orthopedic practice, and the bullet shaped nose of the nail is hammered into the medullary canal. After the nail is completely driven into the bone, the cloverleaf cross-section of the nail, coupled with the lateral resilience afforded by the longitudinal slot, effectively anchors the nail into the cortex of the proximal bone fragment, and in most instances also anchors the distal portion of the fracture against rotational movement. This feature is most important, since any rotational movement between the distal and proximal bone fragments during the healing process will non-align the reduction of the bone fracture and result in a malunited bone which may have to be corrected surgically.
While the Kuntscher cloverleaf nail is often capable of maintaining proper bone fragment alignment during the healing process, there are instances where it can fail. For example, if the fracture is located well into the distal portion of the bone where the intramedullary cavity rapidly flares out in cross-section, the distal end of the cloverleaf nail may fail to gain sufficient purchase on the flared surfaces of the bone cortex.
Heretofore, the prior art has attempted to solve this problem by designing special intramedullary nails having round cross-sections and which abandon the cloverleaf cross-section which renders the Kuntscher nail so effective and popular, and have switched over to a nail having a circular cross-section which relies almost entirely upon a distally located expanding mechanism to gain purchase in the cortex of the distal bone fragment. In one such design, the distal end of the nail is split into several sections along its longitudinal axis, and a conical member which rides on an axially disposed, threaded rod spreads the split ends of the nail apart when the rod is rotated. While this nail is capable of gaining purchase on the distal end of a bone fracture, this design is not without shortcomings. In addition to gaining poorer purchase on the proximal bone fragment as a result of the circular in lieu of the split cloverleaf cross-section, on or more of these split ends can break off when extended by the conical member. Furthermore, when it is necessary to remove the nail after the bone is healed, this design relies solely on the restorative resiliency of the split nail ends to retracted the expanded portion of the nail; a small piece of bone fragment lodged between the threaded rod and one or more of the split nail ends could frustrate the nail removal process. Additionally, because the lateral or transverse extension of the split nail ends is a nonlinear function of the degree of rotation of the threaded shaft, it is difficult for an orthopedic surgeon to determine with precision the degree to which the nail end has expanded by the number of wrench turns on the rod. Finally, because the longitudinal dimension of the nail contracts as a function of the nail end expansion, it is difficult for an orthopedic surgeon to tell, without numerous fluoroscopy readings, exactly where and how the nail is seated on the bone cortex.
Another approach to solve the anchoring problem associated with conventional Kuntscher cloverleaf nails has been to design a nail having a circular cross-section and blades or spikes which can extend out the sides of the distal portion of the nail. However, like the "split end" nails, the abandonment of the resilient cloverleaf cross-section in favor of a circular cross-section impairs the ability of the nail to gain purchase on the cortex of the proximal bone fragment. Additionally, the blades or spikes in some of these nails extend from their lateral ports or slots in a pivoting motion, as opposed to a straight lateral motion. Such pivoting actions are often controlled by cam mechanisms in which there is no simple linear relationship between the amount of transverse extension of the blades or spikes, and the number of turns on the threaded rod. Sucn nonlinearity, of course, makes it difficult for the orthopedic surgeon to easily determine the degree of expansion that the blades or spikes of the device have undergone as a result of the number of wrench turns he has applied to the threaded rod of the device. Moreover, while some of these mechanisms do not rely entirely upon the resiliency of the blade material to retract the blades, the retracting mechanisms employed are unduly complicated, rely on close tolerances in order to function effectively, and are not designed to be retrofitted in any conventional hollow, intramedullary nail.
Clearly, a need exits for a simple, effective expansion device having a set of laterally extendible blades which can be transversely expanded or retracted with precision as a simple function of the number of turns of an elongated control member, and which may easily be retrofitted onto a Kuntscher cloverleaf nail in order to retain the advantages associated with a split cloverleaf cross-section. Ideally, such an expansion device should be simple and inexpensive in construction, and should not rely upon close tolerances in its fabrication in order to afford near-perfect reliability in operation.